Delivery of active pharmaceutical ingredients (APIs) to the eye may be achieved most conveniently by topical administration to the eye. However, topical delivery of APIs to the eye is commonly limited by a number of factors, including low residence time, poor penetration and delivery to target tissue, and physiological barriers to delivery. These limiting factors are particularly significant for diseases affecting the posterior segment of the eye, and for this reason it is common to resort to other routes of administration, e.g., intraocular (intravitreal) injection and systemic administration, to deliver APIs to the posterior segment of the eye.
Of course, intravitreal injection is invasive and requires highly specialized conditions including performance by an ophthalmologist in an operating room. Moreover, intravitreal injection carries attendant risks of infection and, in the case of intravitreal inserts, displacement.
On the other hand, systemic administration, e.g., intravenous injection, while less demanding technically, is subject to physiologic barriers to success. For example, the blood-retina barrier (BRB), like the blood-brain barrier (BBB), limits APIs from reaching the interior of the eye. Moreover, systemic administration may require unacceptably high dosages of API in order to achieve efficacious drug levels within the eye.
Microemulsions are thermodynamically stable and isotropic formulations composed of a polar phase (e.g., water), a non-polar phase (e.g., oil), surfactant, and co-surfactant. Unlike nanoemulsions, microemulsions form without the need for input energy; they form essentially spontaneously. Certain microemulsions, characterized by their clarity, stability, and the possibility of sterilization, represent candidate topical delivery platforms for APIs directed to the eye.
The eye is divided into two anatomical compartments, called the anterior chamber and the posterior chamber. The smaller anterior chamber includes all structures including and anterior to the lens, e.g., the cornea, aqueous humor, iris, and the lens. The much larger posterior segment includes all remaining structures, i.e., all structures posterior to the lens. These structures include, inter alia, the vitreous humor, retina, retinal blood vessels, macula, choroid, part of the sclera, and optic nerve.